develop a kernel module to manage a proprietary protocol
If your protocol use a standard serial line, why do you want to make a real driver just for the protocol. Embed it into a user library which make the job will be:
-easier than developing a driver
-easier to maintain
-less dangerous for the system stability in case of bug or error
The only one positive point I can see for developing a driver is performance purpose.
By the way, if you're thinking that a driver is the way to go, take a look here http://lwn.net/Kernel/LDD3/